1. Technical Field
This invention relates generally to the field of cathodic protection systems for steel-reinforced concrete structures, and is particularly concerned with the performance of cathodic protection systems utilizing discrete anode elements embedded in cementitous grout or mortar.
2. Description of the Prior Art
The problems associated with corrosion-induced deterioration of reinforced concrete structures are now well understood. Steel reinforcement has generally performed well over the years in concrete structures such as bridges, buildings, parking structures, piers, and wharves, since the alkaline environment of concrete causes the surface of the steel to "passivate" such that it does not corrode. Unfortunately, since concrete is inherently somewhat porous, exposure to salt results in the concrete over a number of years becoming contaminated with chloride ions. Salt is commonly introduced to the concrete in the form of seawater, set accelerators or deicing salt.
When the chloride contamination reaches the level of the reinforcing steel, it destroys the ability of the concrete to keep the steel in a passive, or non-corrosive state. It has been determined that a chloride concentration of 0.6 Kg per cubic meter of concrete is a critical value above which corrosion of steel can occur. The products of corrosion of the steel occupy 2.5 to 4 times the volume of the original steel, and this expansion exerts a tremendous tensile force on the surrounding concrete. When this tensile force exceeds the tensile strength of the concrete, cracking and delaminations develop. With continued corrosion, freezing and thawing, and traffic pounding, the utility or the integrity of the structure is finally compromised and repair or replacement becomes necessary. Reinforced concrete structures continue to deteriorate at an alarming rate today. In a recent report to Congress, the Federal Highway Administration reported that of the nation's 577,000 bridges, 226,000 (39% of the total) were classified as deficient, and that 134,000 (23% of the total) were classified as structurally deficient. Structurally deficient bridges are those that are closed, restricted to light vehicles only, or that require immediate rehabilitation to remain open. The damage on most of these bridges is caused by corrosion of reinforcing steel. The United States Department of Transportation has estimated that $90.9 billion will be needed to replace or repair the damage on these existing bridges.
Many solutions to this problem have been proposed, including higher quality concrete, improved construction practices, increased concrete cover over the reinforcing steel, specialty concretes, corrosion inhibiting admixtures, surface sealers, and electrochemical techniques such as cathodic protection and chloride removal. Of these techniques, only cathodic protection is capable of controlling corrosion of reinforcing steel over an extended period of time without complete removal of the salt contaminated concrete.
Cathodic protection reduces or eliminates corrosion of the steel by making it the cathode of an electrochemical cell. This results in cathodic polarization of the steel, which tends to suppress oxidation reactions (such as corrosion) in favor of reduction reactions (such as oxygen reduction). Cathodic protection was first applied to a reinforced concrete bridge deck in 1973. Since then, understanding and techniques have improved, and today cathodic protection has been applied to over one million square meters of concrete structure worldwide. Anodes, in particular, have been the subject of much attention, and several types of anodes have evolved for specific circumstances and different types of structures.
One type of anode that has been utilized for cathodic protection of reinforced concrete structures is catalyzed titanium. The most common configuration of catalyzed titanium anode has been a highly expanded mesh of chemically pure titanium, which is catalyzed by a thin surface coating of precious metal or mixed metal oxides. The anode mesh strands are generally less than about 11/3 inches (3 centimeters) apart. This type of anode has been especially successful for protection of reinforced concrete decks, in which case the anode is fastened to the top concrete surface and overlaid by typically 1-4 inches (2.5-10 centimeters) of fresh concrete. This is known as a distributed anode system since it essentially covers the entire surface of the structure being protected. The fresh concrete overlay serves both to encapsulate the anode and provide a new riding surface for the concrete deck. Approximately 10,000,000 square feet (100,000 square meters) of catalyzed titanium anode have been installed in this way, and such systems have generally provided a long and trouble-free service life.
Another form of catalyzed titanium anode that has been used extensively consists of a pure titanium ribbon, 0.5-0.75 inch (1.25-1.9 centimeters) wide by typically 250 ft (63 meters) long, which is also catalyzed by a thin surface coating of precious metal or mixed metal oxides. The titanium ribbon anode may be flat, or more commonly, it may be expanded to increase surface area and provide a better bond to the concrete. This type of ribbon mesh anode has typically been installed in slots 0.5 inch (1.25 centimeters) wide by 0.75 inch (1.9 centimeters) deep, cut into the top surface of a concrete deck. The slots are typically spaced 12 inches (30.5-centimeters) apart. This type of cathodic protection is therefore referred to as a "slotted" system. After the catalyzed titanium anode has been placed in the slot, the slot is backfilled with a cementitous grout or mortar to encapsulate the anode ribbon and provide a flat riding surface. This type of slotted system has been particularly advantageous for cathodic protection of reinforced concrete parking garage decks, since it can be installed without loss of headroom in the garage and without imposing additional dead weight on the structure.
However, these slotted systems have not been generally successful. After a period of use, the grout in the slots becomes stained by an acidic liquid, and the grout appears dark and wet. This acidic liquid attacks the cement paste and causes deterioration of the grout or mortar surrounding the anode. In extreme cases, this liquid has completely destroyed the grout, leaving the anode fully exposed. In other cases, the liquid has damaged and penetrated the concrete deck. Such attack has caused the voltage of the cathodic protection system to rise, and in time adequate protective current could not be supplied within the compliance voltage of the power supply. It has been speculated that such failures have occurred in non-distributed slotted systems because the cathodic protection current is confined to a relatively small area, thus concentrating the acidic anode reaction products to a small volume of concrete grout. This is in contrast to the more successful highly expanded mesh anode, which effectively distributes the current and the anodic reaction products over a much larger area.
The catalyzed titanium ribbon anode has also been used in another type of non-distributed system or discrete anodes. In this case, a hole, typically 0.75-1 inch (1.9-2.5 centimeters) in diameter and 6-24 inches (15.2-61 centimeters) long is drilled into the concrete member. The hole is filled with cementitous grout or mortar and the catalyzed titanium anode is inserted into the fresh grout or mortar. This system is claimed to be advantageous for larger concrete members such as columns and beams.
But these discrete anodes suffer the same problems as slotted systems. The acidic anode reaction products are confined to a relatively small area surrounding the anode, and eventually cause damage to the cementitous grout or mortar, which in turn causes the system voltage to escalate. The exact cause of this phenomenon is not known, but is generally thought to be due to acidic destruction of the cement paste surrounding the anode followed by a rapid escalation in resistance near the anode-grout interface. Failures are not believed to be due to anode drying out, as the anode-concrete interface can be observed to remain wet.